1. Field of the Invention
The present invention relates to magnetic disks utilized for data storage and in particular to sector servo patterns written on the magnetic disks by the same write head as is used to record data thereon.
2. Discussion of the Prior Art
The methods and processes for storing information on a magnetic disk are well known in the art. Also well understood are the methods and associated hardware used to recover the information stored on a magnetic disk and to process this information into signals useful by equipment such as digital computers. In recovering the information, two factors are essential. First, a read head of a disk drive holding the disk from which the information is to be recovered must be positioned over the proper track on the disk at which the desired data to be recovered is stored. Second, modern efficient data storage techniques allow each track to contain more than one segment of data thus requiring the proper segment to be located. Several methods of accomplishing track and segment location have been practiced in earlier systems.
One such system locates the start of a data segment by writing a pattern on the disk which starts with an erased gap followed by a preamble of written zeroes followed in turn by sector and track identifier codes. Servo position information, used to maintain the read head centered on the track, is then written subsequent to the sector and track identifier codes. This servo position information consists of an "A" burst written off track by one-half the separation between adjacent tracks on the disk and a "B" burst written off track by one-half the distance separating the data track of interest and the next adjacent track to the side opposite that used for the "A" burst. The data segment, written on track, follows the "B" burst. Position information to control a servo mechanism driving the read head, to adjust its position over the proper track, is derived by integrating a half wave rectified waveform from both the "A" and "B" bursts. The integrated amplitude is stored in a sample and hold circuit and is then summed to form the control signal amplitude (A+B). An analog voltage resulting from the signal processor and corresponding to the signal difference (A-B) is fed to a positioning summing junction. A linear motor is driven by the output of the summing junction to reposition the head so as to keep both bursts equal in amplitude. An example of this form of sector servo pattern is shown in FIG. 1 of the drawings herein. A variant embodiment of this system uses a peak detector instead of integrating the signal amplitudes of the "A" and "B" bursts. This alternate embodiment is very sensitive to the amplitude of the largest pulse within the bursts.
The preceding system is useful in that it is self-triggering, that is, the erase gap is used to locate the pattern on the disk. Additionally, the pattern contains track identification information as well as the position information. The pattern is written as a series of three "macro" steps. Referring to FIG. 1, an "A" burst is first written offtrack. The write head is then positioned on track and the erase gap preamble and identification codes are written. Finally, the write head is repositioned an additional one-half track and the "B" burst is written. Data is then entered by returning the write head to the "on track" position. Accurate timing can be achieved by detecting the transition from the erase gap to the preamble, which establishes the time for the end of the "B" burst.
This system generally requires that extra care be taken when writing the identification information so that it can be read "off track". In the described embodiment, two write heads are utilized. One write head serves to write a clock track to identify the start of the erase gap and the other write head, positioned so that the clock track overlaps the written codes, writes the pattern as described above. Both the integrating and peak detector processing schemes utilized in the above result in analog information which requires extensive circuitry, including analog to digital conversion means, to enable the information to be used by a data processor. This results in high circuit complexity, large space requirements for circuit boards, and high cost.
A differing servo pattern has been proposed as given by the ANSII proposal X3B7/1983-B, January, 1983. This pattern is depicted in FIG. 2 of the drawings herein. It is described as a pattern having a preamble of 29 bytes each of FF (base 16) followed by a single byte of FE (base 16). This preamble is followed by 15 bytes of 84 (base 16), termed the "up count", followed in turn by 15 bytes of 82 (base 16), termed the "down count." Two additional bytes of FF (base 16) and then two bytes written as blanks complete the pattern. The start of the pattern is located by a mechanical index placed on a armature plate and sensed by appropriate transducer means. The preamble supplies a synchronizing clock signal for the decoding of the pattern. The up counts and the down counts are self-identifying and each subsequent byte is displaced slightly farther off track than the preceding byte with the first up count byte being written on track. During the reading of the up counts and down counts, the amplitude of each up or down byte is compared to a reference using a single comparator circuit. When the read head is substantially following the center of the track, the count of the up bytes having an amplitude greater than the reference should be equal to the count of the down bytes having an amplitude greater than the reference. Any difference in count thus directly provides digitally coded information for track position. The start of the synchronizing preamble clock pattern provides an accurate timing point to establish the end of the servo pattern and the start of the data. The pattern can be self "write protected" by the mechanical index.
The pattern depicted in FIG. 2 requires an accurate location of a sensor which detects the mechanical index relative to the locations of the write head and read head. This pattern further requires that the write head have a microstepping capability to 1/16th of a track position. Additionally, a clock track timing reference must be given to write the pattern. Since there is only one mechanical index, only one pattern can be written per revolution. This further introduces potential error in reading a pattern written on a different drive having differing mechanical relationships. A further disadvantage can be observed in that relatively complex decoding circuitry is necessary to digitally identify the up and down counts.